1. Field of the Invention
The present invention relates in general to an automatic transmission for an automotive vehicle, and more particularly to a technique for simplified control of the transmission to effect shifting operations while preventing a shifting shock due to temporary disconnection of the transmission line.
2. Discussion of the Prior Art
As an automatic transmission for an automotive vehicle, there is known a hydraulically operated constant-mesh transmission which includes: an input shaft coupled to an engine of the vehicle via a primary clutch; an output shaft connected to drive wheels of the vehicle; a plurality of pairs of gears for transmitting power from the input shaft to the output shaft at different speed reduction ratios; a hydraulically operated actuator device for activating shift members to select one of the pairs of gears. In this automatic transmission, power produced by the engine is transmitted to the drive wheels at the speed reduction ratio which corresponds to the pair of transmission gears automatically selected by the appropriate shift member. Namely, the transmission system indicated above utilizes a conventional manually operated constant-mesh transmission, and a hydraulic shift mechanism for automatically controlling the constant-mesh transmission. This type of transmission permits high fuel economy of the engine, owing to the use of the constant-mesh transmission.
An automatic shifting operation in the transmission system of the type indicated above is effected in a predetermined sequence of operation similar to a sequence of manual shifting operation usually employed by the vehicle driver for a manually operated transmission. More specifically described, when the transmission is shifted from one position to another, the engine output is temporarily lowered by reducing a fuel supply to the engine, and at the same time the primary clutch is disengaged for disconnecting the input shaft of the transmission from the engine. Then, the hydraulic actuator is operated to move the appropriate pair of transmission gears into position. Subsequently, the primary clutch is restored to the engaged position. Since the primary clutch is temporarily disengaged, the transmission of power from the engine to the drive wheels is interrupted for the corresponding period. This interruption gives the vehicle driver an unpleasant shifting feel. The shifting feel is deteriorated, particularly when the transmission is shifted between low-gear positions, for example, upon a shift-up action from the first-speed to the second-speed position, or from the second-speed to the third-speed position, or upon a shift-down action from the second-speed or third-speed position to the first-speed or second-speed position. In these low-gear positions, there exists a relatively large difference in the drive torque between the input and output shafts of the transmission, and consequently the interruption of the power transmission causes a shifting shock.
To minimize such a shifting shock due to the temporary disengagement of the primary clutch upon shifting of the transmission, it is proposed to use a secondary clutch as disclosed in British Patent Specification GB-A-8,203,394 (U.S. Pat. No. 4,544,057) wherein outer and inner input shafts of the transmission are connected to the engine by means of the primary and secondary clutches, such that the primary and secondary clutches are disposed between the engine and the output shaft, in parallel with each other. While the primary clutch is temporarily disengaged to allow for a shifting operation of the transmission, the secondary clutch is placed in partially engaged position, in order to transmit a rotary motion of the engine to the drive wheels, and thereby avoid the complete disconnection of the engine from the drive wheels even during the shifting action of the transmission.
In such a conventional transmission using the parallel primary and secondary clutches, the secondary clutch must be partially engaged at the moment when the primary clutch is disengaged upon a shifting operation of the transmission. This requires a complicated control arrangement for intricate timing control of the disengaging and engaging actions of the primary and secondary clutches. If the engaging action of the secondary clutch takes place too early relative to the disengagement of the primary clutch, the transmission is brought to a locked state, causing a considerable amount of shifting shock. If the secondary clutch is engaged too late, the engine is subject to a racing phenomenon due to a sudden drop of the engine load after the disengagement of the primary clutch. The exact control of the disengaging and engaging actions of the two clutches has been considered difficult.